1. Field
The present disclosure relates generally to semiconductor processing equipment and, more particularly to a flow ratio controller for delivering contaminant-free, precisely metered quantities of process gases in predetermined ratios to at least two locations of a process tool or tools. More particularly, the present disclosure relates to a system for and method of dividing flow from a single gas box in predetermined ratios to four locations of a process tool or tools with antisymmetric optimal performance.
2. Background
As used herein the term “gas(es)” includes the term “vapor(s)” should the two terms be considered different.
The fabrication of semiconductor devices often requires the careful synchronization and precisely measured delivery of as many as a dozen gases to a process tool, such as a vacuum chamber. Various recipes are used in the fabrication process, and many discrete processing steps can be required, where for example a semiconductor device is cleaned, polished, oxidized, masked, etched, doped, or metalized. The steps used, their particular sequence and the materials involved all contribute to the making of particular devices.
Accordingly, wafer fabrication facilities are commonly organized to include areas in which chemical vapor deposition, plasma deposition, plasma etching, sputtering and other similar gas manufacturing processes are carried out. The process tools, be they chemical vapor deposition reactors, vacuum sputtering machines, plasma etchers or plasma enhanced chemical vapor deposition chambers, or any other device, apparatus or system, must be supplied with various process gases. Pure gases must be supplied to the tools in contaminant-free, precisely metered quantities.
In a typical wafer fabrication facility the gases are stored in tanks connected via piping or conduit to a gas delivery system. The gas delivery system includes a gas box for delivering contaminant-free, precisely metered quantities of pure inert or reactant gases from the tanks of the fabrication facility to a process tool and/or chamber. The gas box typically includes a plurality of gas flow lines each having a flow metering unit, which in turn can include valves, pressure regulators and transducers, mass flow controllers, filters/purifiers and the like. Each gas line has its own inlet for connection to a separate source of gas, but all of the gas paths converge into a single outlet of the gas box for connection to the process tool.
Sometimes it is desirable to divide or split the combined process gases provided at the outlet of a gas box so that they can be delivered to multiple locations of a single process tool or among multiple process tools. In such cases, the single outlet of the gas box is connected to the multiple locations through secondary flow lines or channels. In some applications, where, for example, the upstream pressure needs to be kept lower than atmospheric pressure (e.g., kept below 15 PSIA) for safety or other reasons, a flow ratio controller is used to insure that the primary flow of the outlet of the gas box is divided in accordance with a preselected ratio among the secondary flow paths or channels.
Flow ratio controller systems of the type shown in U.S. Pat. No. 6,766,260 will stabilize to the desirable ratio split after being initially set, but flows take time to stabilize, and in some applications this can be unsatisfactory. Further, the pressure drop across the valves of the flow ratio controller system can be significantly high. Also, the controller system provides poor control performance for handling downstream blocking of one of the secondary flow paths. Additionally, the system can be difficult to set up because of difficulties in initially determining fixed valve positions of the valves in the secondary flow lines. And for current embodiments using two secondary flow lines it is necessary to assign the high flow valve as the fixed valve and the low flow valve as the controlled valve for flow ratio control.
One application for a flow ratio controller system is to control the flow of gas to a “shower head”, a fixture used in the process chamber of a process tool for making semiconductor devices, such as described in U.S. Pat. No. 7,072,743. Shower head fixtures each include two gas outlets, one from opening(s) provided in the center (or inner portion) of the fixture, and one from opening(s) provided around the periphery (or outer portion) of the fixture. Flow through opening(s) in the outer portion of a shower head fixture affects the outer portion or zone of a wafer being created in the chamber, while the flow through the opening(s) in the inner portion affects the inner portion or zone of the wafer being created. Greater flow to the outer zone than the inner zone is desirable to provide an even application of gas to the wafer being processed. Thus, carefully controlling the ratio of the gas flow provided from the inner portion relative to the gas flow to the outer portion results in the even application of gas to the wafer.
An improved flow ratio controller system, referred to as a two channel or DAO flow ratio controller system, is described in U.S. Pat. No. 7,621,290, assigned to the present assignee. The embodiment described in the patent utilizes a dual antisymmetric optimal (DAO) control algorithm for dividing a single mass into two flow lines. Each flow line includes a flow meter and a valve. Both valves of the flow ratio controller system are controlled through a ratio feedback loop by a DAO controller. The latter includes a single input, single output (SISO) controller, an inverter and two linear saturators. The output of the SISO controller is split and modified before being applied to the two valves. The two valve control commands are virtually antisymmetric to the maximum allowable valve conductance position as one of the two saturation limits. This means that one of the valves is kept at its maximum allowable valve conductance (opened) position at any moment of time while the other is actively controlled to maintain the flow ratio. This results in the DAO flow ratio controller contributing the minimum pressure drop to the gas flow. One application for the DAO flow ratio controller system is controlling the flow ratio to each of the inner and outer openings of a shower head fixture relative to the total flow through both.
Current production of wafers can include identical wafers being simultaneously manufactured in separate chambers, with the process for each chamber being identical. The sequence and flow rate of gases flowing into each chamber will therefore be identical, and thus there would be savings to use an integrated control system including a single gas box that controls the same ratio of gases with each step of the process to each of the chambers. With a single gas box used to provide gases to two chambers, considerable savings, and a smaller footprint is needed for the equipment. Further, with advances, the size of wafers are getting larger requiring more than one shower head fixture to be used in order to insure the gas flow over the entire wafer is properly controlled. However, there is a challenge to ensure that the proper flow ratio is maintained during each step of the process.
One flow ratio controller system for controlling the ratio of the flow of gases through more than two flow lines is described in U.S. Pat. No. 7,673,645. The flow ratio controller utilizes a multiple antisymmetric optimal (MAO) algorithm for dividing a single mass flow into multiple (more than two) flow lines. Each flow line is provided with a SISO feedback controller combined with a linear saturator to achieve the targeted flow ratio set point. Each valve control command is antisymmetric to the summation of all of the other valve control commands so that the MAO control algorithm guarantees that there exists at least one valve at the allowable maximum open position at any moment so that the optimal solution in terms of the maximum total valve conductance for a given set of flow ratio set points is achieved. This approach provides excellent control with minimum pressure drop through each flow line, and can satisfactorily be used for most applications. However, in the example where it is desirable that the ratio of the rate of flow to the inner openings of two shower head fixtures are substantially the same and the ratio of the rate of flow to the outer openings of the two fixtures are substantially the same, all relative to the total rate of flow into the fixtures, the control functions for the flow lines do not necessarily settle with precisely the same response time with each change in flow occurring though the flow lines, resulting in imprecise control.
One approach to providing the same ratio to the inner and outer openings of two shower head fixtures is shown in FIG. 1. The hardware system includes three two channel flow ratio controller units 10A, 10B and 10C, each of the type described U.S. Pat. No. 7,621,290, arranged in two stages. The first stage includes one unit 10A that receives the total flow at a rate of Qt from a gas box or other source, and initially divides the total flow at rate Qt into two flows at rates Qa and Qb (where Qt=Qa+Qb) in accordance with a ratio set by the host controller 12. The second stage includes two controller units 10B and 10C. Unit 10B receives the total flow at a rate of Qa from controller unit 10A, and divides the total flow at rate Qa into two flows at rates Q1 and Q2 (where Qa=Q1+Q2) in accordance with a ratio set by the host controller 12. Similarly, unit 10C receives the total flow at a rate of Qb from controller unit 10A, and divides the total flow at rate Qb into two flows at rates Q3 and Q4 (where Qb=Q3+Q4) in accordance with a ratio also set by the host controller 12. The two stage control is used to control the flow of gas to two shower head fixtures (not shown) both at the same ratio so that the Qa=Qb, Q1+Q2=Q3+Q4, and the ratio Q1/(Q1+Q2)=Q3/(Q3+Q4). This result in the ratio of flow to the inner and outer portions of each shower head fixture being the same. Each controller unit 10 includes it's own controller 14 for controlling the valves 16 in response to the corresponding sensor signals generated by the flow sensors 18 so as to maintain the programmed ratio provided by the host controller 12. Host controller 12 thus must be used to coordinate the three hardware units 10. While the arrangement works, there is an increase in pressure drop and a reduction in the optimal valve conductance along each flow path because of the two stage approach.
Accordingly, it is desirable to provide a flow ratio controller system that provides relative flow rates through four flow lines with minimum pressure drop through each flow line, with antisymmetrical optimal control, and with substantially the same response times for all of the flow lines.